1. Field of the Invention
The present invention provides an improved motion control method and apparatus for a flat bed scanner. More specifically, the present invention provides a method for eliminating backtrack artifacts and scanning errors associated with backtracking operations.
2. Description of the Related Art
Scanners are used to scan a target image and create scanned image data which can be displayed on a computer monitor, which can be used by a computer program, which can be printed, which can be faxed, etc. Scanned data may be saved to memory or a magnetic or optical drive, or other fixed or removable memory device. Scanning devices may be packaged in a stand-alone housing or as part of a multi-function peripheral, including a printing component to perform scanning as well as standard copying functions.
Scanners typically include a housing aperture defined by an edge wherein a platen is located. A target document is positioned on the platen for scanning of the text or image by a scanbar. Depending on the positioning of the scanbar relative to the platen, the platen may be transparent where the scanbar is beneath the platen or may be solid where the scanbar is above the platen. For a typical flatbed scanner, the scanbar will be below the platen, which will have a transparent section to allow for the scan operation.
In a conventional scanning operation the scanbar starts out in a home position as determined by an origin or home reference. By having the scanbar locate the scanbar home reference before starting a scan, the scanner is able to establish an accurate position reference for the scanbar each time the scanbar moves from the scanbar home. In addition, the scanbar will also calibrate the sensor elements to a known white (or other color) surface and, optionally, to a known black surface before performing a scan of the image. In some instances a prescan is performed. A prescan of the image is performed by moving the scanbar along the major axis (typically, the length of the platen aperture area) in relation to or over the image for the length of the platen. The scanner obtains information about the image from the prescan which the scanner uses for the subsequent image scan of the image. If a prescan is performed, the scanbar is returned to the scanbar home reference to reestablish an accurate position for the scanbar. An image scan is performed by first moving the scanbar to a constant velocity and then continuing the scanbar at that constant velocity along the major axis in relation to or over the image. After the image scan, the scanbar is returned to the scanbar home reference to reestablish an accurate position for the scanbar and to await the next image. In other instances the prescan method is omitted.
Conventional flat bed scanners use either a stepper motor in an open loop control system or a DC motor in a closed loop system to move the scanbar in a scan direction to perform the prescan and image scan. The scan direction is defined generally as the direction of motion of the scanbar and typically parallels a major, or longer, axis of the platen. A closed loop circuit is a circuit in which the output is continuously fed back to the controller for constant comparison. An open loop circuit, on the other hand, is a control system that does not have means of comparing input and output for control purposes. Regardless of which control system is utilized, a backtracking operation may need to be performed. During a flatbed scan, image data is temporarily stored in an image data buffer. Depending on image content and data transfer rates, the image data buffer may become filled with image data. This happens when image data is coming into the data buffer faster than the image data is processed out of the image data buffer. When a data buffer is full or nearly full of image data, the buffer must empty completely or below a threshold level before the scanbar can resume acquiring scan data. Specifically, the scanbar will stop, reverse direction, or backtrack some pre-selected distance, and stop at some position where data has already been taken. When the buffer level reaches a threshold level that allows for scanning to continue, the scanbar will accelerate to a constant velocity in the scan direction, and resume taking data at a point immediately after the last valid data location.
In either the DC motor closed loop system or the stepper motor open loop system there will be location errors associated with the backtracking operation. In some instances data is duplicated or left out causing backtrack artifacts. Artifacts are image errors in the scan or copy output that are noticeable to the typical user, that appear as dark or light spots or lines in the image output. The artifacts are due to the difference in the actual physical location of the scanbar and location where the processor estimates the scanbar is located. Such positional errors may arise due to mechanical positional tolerances present in the prior art devices. For example, DC motors are typically used in combination with an encoder wheel and a photo interrupter, which detects rotations of the rotary encoder to control the speed of the scanbar and determine location of the scanbar. However, noticeable scan errors arise from differences between actual location and estimated location of the scanbar and are often generated during backtracking steps when the control system assumes that the scanner is at a new location or when a slice of data is skipped because of scanner positioning errors. Such backtracking errors and artifacts are highly undesirable.
Given the foregoing, it will be appreciated that a method is needed which eliminates scanning errors and backtrack artifacts associated therewith.